The present invention relates to a resetting circuit of a key telephone subset (hereinafter referred to as KTS) in a key-phone system and, more particularly, to a resetting circuit capable of preventing data error and power-down phenomenon caused by unstable AC power supply which is provided from a key system unit (hereinafter referred to as KSU).
Generally, a key-phone system includes a power supply for the KSU to supply the KTS with +24V through sixteen extension lines, when the sixteen extension lines are connected to eight office lines. Further, the KTS adjusts the power supplied from the KSU to an adequate power which is suitable to operate the KTS and internal circuit thereof. Therefore, the KTS includes self power supply for the purpose of supplying stable power.
Referring now to FIG. 1 illustrating a conventional resetting circuit, there is KSU 100 connected to a portion 200 of the KTS. A power supply 10 of the KSU 100 supplies the KTS with +24V through a power port P1 and P2. Said power +24V is applied to capacitors C1 and C2, and a first and second switching power circuits 11 and 12 to generate a first voltage Vcc1 (+12V) and a second voltage Vcc2 (+5V). Then, said Vcc1 and Vcc2 generated by the first and second switching power circuits 11 and 12 are applied to a microprocessor 20 of the KTS 200, wherein Vcc1 is applied to the microprocessor 20 via a data link 30 of the KTS and Vcc2, delayed by a time constant which is determined by resistor R11 and capacitor C11, is then applied to a reset terminal RST of the microprocessor 20, causing microprocessor to be reset. If the AC power of the power supply 10 is off instantaneously during the operation, then the Vcc1 falls in abnormal state while the Vcc2 keeps in a normal state, of course, it depends on the power-off timing and power load of the power supply 10.
Accordingly, though the microprocessor 20 keeps working, the Vcc1 which is applied to the data link 30 is downed to 2 or 3 voltages. Hence, data logic is changed, by which the microprocessor may read and send error data. Therefore, it is a drawback of the prior key-phone system that the microprocessor 20 of the KTS is power-downed due to said reasons describe above. Further, a displaying means (not shown in FIG. 1) of the KTS 200 displays an error message thereon.